Field of the Invention
The present invention is directed to a method and apparatus for calibrating image data from a given medical imaging protocol.
Description of the Prior Art
In the medical imaging field, several imaging schemes are known. For example PET (Positron Emission Tomography) is a method for imaging a subject in 3D using an injected radio-active substance which is processed in the body, typically resulting in an image indicating one or more biological functions.
The Standardized Uptake Value (SUV) is a widely-used measure for quantifying radiotracer (especially 18F-FDG) uptake in clinical PET scans. This value is computed from the number of counts of emission events recorded per voxel in the image reconstructed from event data captured in the PET scan. Its use is intended to provide normalization for differences in patient size and body composition, along with the dose of radiotracer injected, thereby enabling inter-study comparison, both between and within individual patients.
While raw scan data may be expressed in units of Bq/ml, SUV is calculated as:
      scan    ⁢                  ⁢    data    ⁢                  ⁢          (              in        ⁢                                  ⁢        Bq        ⁢                  /                ⁢        ml            )        ×    patient    ⁢                  ⁢    mass    ⁢                  ⁢          (              in        ⁢                                  ⁢        grammes            )            total    ⁢                  ⁢    injected    ⁢                  ⁢    dose    ⁢                  ⁢          (              in        ⁢                                  ⁢        Bq            )      
This is typically simplified by assuming that the patient has a density of 1 g/ml, in which case the SUV becomes dimensionless.
SUVmax is the maximum observed value of SUV within a region of interest: typically a three-dimensional volume of interest, for example a representation of a lesion.
While differences in body composition and injected dose represent one source of variation, differences in scanner hardware and reconstruction software represent others, and these are not addressed by the use of SUV.
It has been observed that a single set of raw scan data may result in differing values for SUV, and so also for SUVmax, depending on the reconstruction applied to the raw scan data. A “reconstruction”, in this context, is the treatment applied to a digital photon count to convert it into image data. Practically, it is carried out by a digital computer. For example, a low resolution reconstruction will result in significant “blurring” of the image produced, so that a small lesion may appear to have a lower SUVmax than in reality, while a larger lesion of a same SUV will appear to have a larger, and possibly correct, SUVmax. Using a higher-resolution reconstruction on the same raw data will show images with more clearly-defined edges, in turn meaning that small lesions will appear to have greater SUVmax than under the lower-resolution reconstruction. SUVmax is typically the clinically-reported result of a scan.
It is clearly undesirable for the results of the PET scan to vary according to the reconstruction applied. For example, it may be required to evaluate a patient's progress by comparing two PET scan results taken at different times on different scanners. Each may have a different reconstruction, for example because a newer scanner has a higher resolution capacity. However, the two results must be aligned, that is, made comparable.
The described variations in reported results impede the acceptance of PET as a quantitative imaging tool for lesion characterization, prognostic stratification and treatment monitoring, since differences in scanner hardware and reconstruction can significantly impact generated SUV values. As such, better standardization and improved comparability between scanners and reconstruction protocols are required.
Two reconstructions may be performed on each data set. Typically, this will include one high-resolution reconstruction for visualization, whereby a user may be presented with an image of high quality for visual consideration, while a second lower-quality reconstruction is also performed to provide an image of a standard reference quality to enable it to be compared to images produced by lower-resolution scanners. Effectively, higher-quality image data is blurred or downgraded to provide an image of standard resolution for quantification, allowing the results from a high-resolution scanner to be compared to results from lower-resolution scanners. The higher-quality image is preferably made available for visualization by a user.
U.S. Pat. No. 8,755,574 and the equivalent GB2469569 address this situation, and provide for image calibration by reference to a reference object. This may be a standard “phantom”, such as a NEMA image quality phantom, well known in the art. Imaging of such a reference object allows recovery coefficients of the imaging system to be determined as the characteristics of the phantom are known a priori.
U.S. Pat. No. 8,755,574 and the equivalent GB2469569 describe methods for calibrating image data from a given medical imaging protocol, in particular for aligning PET images acquired using a “preferred” reading reconstruction-protocol to a “reference” reconstruction-protocol. The procedure relies on a phantom calibration step whereby an optimal filter size, which aligns data reconstructed using the “preferred” protocol to a “reference” reconstruction protocol, is estimated. The procedure is based on minimizing the differences in activity concentration recovery coefficients (RCs) calculated from a filtered version of the phantom data reconstructed with the “preferred” protocol and a reference RC curve. The filter that best aligns the RCs can then be used to align clinical PET data acquired using the “preferred” protocol to the “reference” protocol. The “preferred” reconstruction protocol is aimed at image visualization, whereas data aligned to the “reference” reconstruction-protocol (not visible to the user) is used for quantification.
The known method includes: (a) obtaining reference image data from a scan of a reference object using the medical imaging protocol; comparing the obtained reference image data of the reference object to standard reference image data for the reference object, and modifying the obtained reference image data to reduce an error between the obtained reference image data and the standard reference image data; and (b) obtaining subject image data from a scan of a subject using the medical imaging protocol, modifying the subject image data based on the modified reference image data, and obtaining from the modified subject image data a value of a variable for display with unmodified subject image data.
It is necessary to perform at least one phantom scan per phantom model on each scanner; and once for each reconstruction. It has been found to be inconvenient to have to rely on imaging a reference article, such as a phantom. In a real-life clinical setting, a suitable phantom may not be readily available. It is not a trivial task to prepare an imaging phantom for scanning, or to perform the required reconstruction of scan data. A clinical environment may lack the materials, expertise and time to perform phantom scanning for image data calibration.